Tunable transceivers for colorless spectrum-sliced WDM passive optical networks

ABSTRACT

This application discloses apparatuses and methods for selecting and tuning of a select mode of a multi-longitudinal mode device seeded or wavelength locked to a spectrum-sliced external wavelength by either self-seeding or broadband light-source seeding through an array-waveguide grating.

FIELD

The present document relates to the field of colorless passive-opticalnetworks based on dense wavelength division multiplexing (WDM-PON), andmore particularly to longitudinal-mode tuning at the client side ofcolorless WDM-PON based on broadband light source (BLS) seeding orself-seeding (SS). In particular, the application relates to opticaltransmitters that may be used, but are not limited to WDM-PON, and toWDM-PONs comprising such optical transmitters.

BACKGROUND

Access networks are presently experiencing rapid growth around theworld. Both residential and business customers are demandingincreasingly higher bandwidths from their Internet service providers whoin turn are pressed to implement networks capable of deliveringbandwidths in excess of 100 Mb/s per customer. For this application,passive-optical-networks (PON) are particularly well suited as theyfeature lowest capital-equipment expenditures relative to point-to-pointand active optical networks. The books by C. F. Lam, Passive OpticalNetworks: Principles and Practice, Academic Press, 2007, and by L. G.Lazovsky, N. Cheng, W-T. Shaw, D. Gutierrez, S-W. Wong, BroadbandOptical Access Networks, Willey, 2011, and publication by C-H. Lee, W.V. Sorin, and B. Y. Kim, “Fiber to the Home Using a PON Infrastructure”,IEEE J. Lightwave Technol., vol. 24, no. 12, pp. 4568-4583, 2006 givegood introduction into this technology.

A passive optical network allows a plurality of users to be connected toa node of a core network (for example, a metropolitan area network). APON comprises an optical line termination (OLT) located at the centraloffice (CO) and an optical distribution network (ODN) which comprises aplurality of optical links and passive optical components arranged so asto form a point-to-multipoint structure whose root is at the CO of theservice provider. At its far end, each optical link may be terminated bya respective optical network unit (ONU). Namely, not all channels haveto be terminated for the network to work. The ONUs may be located atuser's premises and depending on the location of the optical link endone differentiates Fiber To The Home (FTTH), Fiber To The Building(FTTB), or Fiber To The Curb (FTTC), all commonly referred to as FiberTo The X (FTTX).

In a WDM PON, each ONU communicates with the OLT by using a respectivepair of wavelengths: an upstream wavelength for data transmission fromthe ONU towards the CO and a downstream wavelength for data transmissionfrom CO to the ONU. The wavelengths are generally located on a frequencygrid specified by the International Telecommunications Union, in thiscase Recommendation ITU-T G.694.1. One possible arrangement is that theupstream wavelengths are located in the ITU C band (1531-1570 nm) andthe downstream wavelengths are located in the ITU L band (1571-1611 nm).Other possibilities, in which downstream wavelengths are in the E band(1371-1470 nm), for example, are also possible, depending on thespecific manufacturer or service provider's use of the installed fiberbase. The density of wavelength in the band is specified by the gridfrequency separation and the ITU-T G.694.1 recommendation currentlyspecifies 12.5 GHz, 25 GHz, 50 GHz, and 100 GHz grid, with even largerfrequency separations, such as 200 GHz are possible and have been used.The grid separation indirectly determines the constraints on thetransmitters and receivers used in the OLT/ONU.

The optical components at the ONU end and possibly at the OLT end arefiber-optic transceivers: small packaged optical-electronic modules thatare used to simultaneously transmit and receive optical signals at twodifferent wavelengths. Every transceiver comprises optical componentsdetect and generate optical signals, and the electronics to convertthese signals to/from digital data stream incoming from networkprocessors. Optical transceivers generally use connectors to mate tooptical fibers that provide and take away optical signals. When opticalsignals coming to the transceiver and being transmitted from thetransceiver are encoded on different wavelength, i.e., upstreamwavelength different from downstream wavelength, a single fiber andfiber connector is often used. Such transceiver include a duplexer whichseparates the upstream from the downstream traffic.

In a WDM passive optical network, the ODN typically comprises a “remotenode”, a feeder optical fiber connecting the remote node to the OLT anda number of distribution optical fibers connecting the remote node toindividual ONUs. The feeder fiber lengths vary depending on the serviceprovider's architecture demands and ranges from 20 km to 80 km. Thedistribution fibers typically have a length ranging from some tens ofmeters to several kilometers, depending on the environment (metropolitanor rural) and on the application (FTTX). In the attempt to maintain theaccess network inexpensive, the remote node is typically kept passive sothat no separate power supplies need to be installed to power providedto maintain the network. This means that the remote node contains onlypassive components.

The function of the remote node is to multiplex and demultiplex WDMsignals: The upstream signals from all the OLTs, each with its ownfrequency are combined at an array-waveguide grating in the remote nodeinto a single fiber where all the signals coexist, but are encoded onamplitude modulated signals at different wavelengths. The typical numberof different wavelengths that can be fit into a specific ITU banddepends on the frequency separation (grid). Typical numbers M forcommercial array waveguide gratings are 16 for 200 GHz, 32 for 100 GHz,40, and 48 for lower separations. At the other end of the network, thearray waveguide grating demultiplexes the WDM signal into M signals atdifferent wavelengths and routes these signals to M transceivers.

Array-waveguide grating (AWG) is ubiquitous in optical networking and isused for filtering, separating, combining, and routing signals ofdifferent wavelengths as is well known in the art. Its use and principleof operation is described in publicly available texts, such as, “WDMTechnologies: Passive Optical Components” by A. K. Dutta, N. K. Dutta,and M. Fujiwara, published by Academic Press in 2003. It is well knownin the art today that AWG temperature variation can be efficientlycompensated by using so-called athermal array-waveguide gratings. Thistechnology is described in publicly available texts such as “RecentProgress on Athermal AWG Wavelength Multiplexer” by Shin Kamei publishedat the Optical Fiber Communications conference in San Diego, Calif. in2009.

Wavelength division multiplexing in passive optical networks (WDM-PON)is one of the actively investigated as next-generation optical networkarchitecture. WDM-PON provides higher bandwidth per user than any othernetwork architecture and hence potentially offers the lowest cost perunit of bandwidth to the user. However, the key difficulty in such asystem has been the cost of the components, particularly arising fromthe need to transmit light at one wavelength for a specific channel andalso receive information at any one of several other wavelengths at theuser end in the so-called optical network unit (ONU). WDM optical andoptoelectronic components traditionally exhibit high cost, among otherissues, due to precise wavelength definitions in such systems. Adramatic cost reduction is achieved by eliminating wavelength-specifictransceivers at the ONU in the colorless WDM-PON system, also referredto as a system with wavelength-agnostic transceivers in the ONU.

In a colorless optical network, the wavelengths emitted and received bythe transceiver in the ONU are defined in the remote node or the COrather than in the transceiver at the ONU as is well known in theart—see book by C. F. Lam cited above. One commonly implementedarchitecture uses a broad-band light source (BLS) to providespectrally-sliced spontaneous emission to injection-lock to alongitudinal mode of the gain and modulation chip (GMC) as described inthe book by L. G. Kazovsky cited above. This solution, however, suffersfrom high cost of the broadband light source. Further reduction incomplexity and cost can be realized by using a self-seeding scheme asdescribed in a publication by E. Wong, K. L. Lee, T. B. Anderson,“Directly Modulated Self-Seeding Reflective Semiconductor OpticalAmplifiers as Colorless Transmitters in Wavelength Division MultiplexedPassive Optical Networks”, IEEE J. Lightwave Technol., vol. 25, no. 1,pp. 67-74 (2007).

This self-seeding technique, also referred to as self-tuning techniqueby some authors, relies on locking the GMC emission to a wavelengthspecified externally by spectrally slicing the spontaneous emission fromthe GMC itself, i.e., on injection self-locking. Spectral slicing refersto filtering the GMC spontaneous emission and returning it to the GMCfor amplification. If the round-trip loss of the external cavity formedby the GMC, the fiber, and the external mirror is smaller than theunsaturated gain available in the GMC (at given current), lasingthreshold will be reached. Above threshold all the additional currentfed into the GMC will be converted into light at the emissionwavelength, while spontaneous emission at wavelengths other than theselected emission wavelength will be reduced due to gain clamping.Resonators with such long cavities (several kilometers) behave likelasers in that they exhibit clear threshold in the light-currentcharacteristic, but the linewidth and noise of the emitted light aregoverned by spectral slicing (filtering at the array-waveguide grating)and by thermal nature of the spontaneous emission.

A colorless WDM-PON architecture based on self-seeding is illustratedwith the help of FIG. 1 (PRIOR ART). FIG. 1 shows the network componentsconnected into a network 100 comprising a central office 101, feederfiber 111, remote node 109, distribution fiber 103, and ONU transceiver104. The central office 101 may comprise, in one location, thecomponents from the remote node 109 and the components from transceiver104 connected with some small length of optical fiber instead of thedistribution fiber 103. In other words, the central office may be amirror image of the components used on the client side with thedifference that the wavelengths emitted from the central office 101 (allthe downstream wavelengths Σλ_(D)) are different from the upstreamwavelengths Σλ_(U). The upstream may be located in the C-band, while thedownstream may be in L-band.

The remote node 109 comprises an array-waveguide grating 116 with Mdistribution ports 108 and one common port 112. One of the distributionports is connected to a distribution fiber 103 for illustration. Thecommon port 112 of the AWG 116 is connected to a 45° Faraday Rotator anda mirror, i.e., a Faraday Rotating Mirror configured to reflect aportion of the spectrally sliced spontaneous emission coming from allthe transceivers connected to the distribution ports 108, and transmit aportion to the feeder fiber 111 through a semi-transparent mirror 114.This type of arrangement is published by F. Saliou, P. Chanclou, B.Charbonnier, B. Le Guyader, Q. Deniel, A. Pizzinat, N. Genay(1), Z. Xu,H. Lin titled “Up to 15 km Cavity Self Seeded WDM-PON System with 90 kmMaximum Reach and up to 4.9 Gbit/s CPRI Links” in ECOC 2012 TechnicalDigest, paper We.1.B.6.

It is also possible to use an optical coupler to let a portion of energypassing from the AWG 116 to the trunk fiber and portion to ahigh-reflectivity Faraday Rotating Mirror in the other branch of theoptical coupler. This is not shown in FIG. 1, but is published by Q.Deniel, F. Saliou, P. Chanclou, D. Erasme, titled “Self-Seeded RSOAbased WDM-PON Transmission Capacities” in the Technical Digest of the2013 NFOEC/OSA Conference, paper OW4D.3.

The transceiver connected to the distribution fibers typically comprisea duplexer 110 which separates the upstream and the downstream trafficby directing the downstream optical traffic to the detector andtrans-impedance amplifier 124, while the light emitted from the gain andmodulation chip 105 passes through a Faraday Rotator 115 before reachingthe duplexer 110 and being emitted into the distribution fiber 103. TheGMC 105 is a chip with two facets 107 and 125 in which the back facet107 exhibits high reflectivity coefficient and the front facet 125 avery low reflectivity coefficient. It is also common to curve the GMCwaveguide at the point where it reaches the chip edge to reduce thereflectivity of facet 125 even further. In addition, the light passingthrough the high-reflectivity back-facet reflector 107 is generallycoupled to a monitor photodiode and its signal is used to automaticallycontrol the output power of the GMC chip. The upstream and downstreamdigital data transmission are processed in the digital chip 126, in thefigure referred to as PHY for physical media adapter. Digital data isencoded into the optical signals by modulating the gain of the GMC 105using an electrical signal connected to the PHY chip 126. Amplitudemodulation means that the symbols are encored into the amplitude of thelight being transmitted, such as, low and high power, where theintensity of the output beam is modulated between two values per bit orpulse amplitude modulation (PAM) where more than one power level is usedto encode information. Note that duplexer is commonly referred to as adiplexer in the industry. The term diplexer refers to a frequencymultiplexer with two frequencies in which two signals of differentfrequencies travel in the same direction, whereas the term duplexercomes from “full duplex communication” meaning simultaneousbidirectional signal flow over a single path, which is also realizedwith two different frequencies. Diplexer and duplexer are generallypassive reciprocal devices and hence may physically be identical. Forthe purposes of this application, the term diplexer has the same meaningas the term duplexer.

The principle of light generation involves broadband spontaneousemission generated and intensity modulated by the gain and modulationchip 105 (GMC) which is emitted towards the remote node 109 via thedistribution fiber 103. The spontaneous emission is filtered in the AWG116 and light around a narrow linewidth around one wavelength λ_(U) ispassed through to the Faraday Rotating Mirror 113/114. The reflector 114reflects a portion of the incident light back towards the AWG 116 andfinally to the GMC 105 via the distribution fiber. The existence of twoFaraday Rotators 113 and 115 removes the birefringence in the path ofthe light between the remote node and the transceiver. Transceiver 104often is equipped with a heater 120 which is used to increase thetemperature of the GMC to broaden and shift the gain spectrum of theGMC.

At present time, there is no commercial deployment of self-seededWDM-PON as described above. Numerous technical issues have preventedthis. The main difficulties can be enumerated as originating form (a)the presence of residual modulation in the seeding light which reducesthe link margin, (b) link instability originating from random andtime-dependent birefringence of the fibers and components and the highdegree of polarization sensitivity of the gain medium, (c) noiseinherent in the spectrally sliced light, and (d) large linewidths of theemitted light causing large dispersion penalty.

In addition to the above-mentioned network architecture-related issues,there is a device-related issue that is present today and will be becomemore so a problem in the future development of WDM-PON systems based onBLS-seeding and self-seeding when channel density increases: It isrelated to the matching of the longitudinal modes of the GMC 105 to thechannel of the AWG 116. Namely, the channel wavelength of an athermalAWG 116 is very temperature stable (˜2 pm/° C.), while the longitudinalmodes of semiconductor lasers or amplifiers used in GMC 105 aresignificantly more temperature sensitive; they move at ˜100 pm/° C. Forall ambient temperatures of the remote node 109 and the transceiver 104(note that they are generally different) one has to have at least onelongitudinal mode of the GMC fall within the bandwidth of the AWG 116channels to achieve high output power necessary for efficient operationof the spectrally-sliced system. In order to achieve stable operation inthe external cavity with varying ambient temperature and external cavityloss/dispersion fluctuation, the approach taken in the industry has beento (a) minimize the front facet reflectivity on the GMC 105 and (b) uselong gain chips whose mode separation is sufficiently small inwavelength so that at least one GMC mode always appears in the AWGpassband regardless of temperature and drive current.

The difficulty with both (a) and (b) is increased cost. Reflectivitieslower than 10⁻⁴ requires precision coating and angled waveguides, whilelonger chips consume more of the wafer area. Typical Fabry-Pérot lasersare 0.5 mm long, while for channel pitch below 100 GHz, the Fabry-Pérotlasers will have to become longer than 2 mm to ensure that more than onemode always appears in the AWG channel passband.

Therefore, an unmet need for a low-cost high-performance WDM-PONsolution exists in the industry. This application discloses apparatusesand methods of resolving the device related issue to create acost-effective WDM-PON solution.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1: (PRIOR ART) Schematic illustrating a self-seeded WDM-PON system.

FIG. 2A: One embodiment of transmitter optical source employing GMC modeto AAWG channel tuning based on sensing the laser drive current I_(LD).The transmitter is based on self-seeding with modulation averaging.

FIG. 2B: One embodiment of transmitter optical source employing GMC modeto AAWG channel tuning based on maximizing the return power P₁. Thetransmitter is based on self-seeding with modulation averaging.

FIG. 3A: One embodiment of transmitter optical source employing GMC modeto AAWG channel tuning based on sensing the laser drive current I_(LD).The transmitter is based on self-seeding with Faraday rotators.

FIG. 3B: One embodiment of transmitter optical source employing GMC modeto AAWG channel tuning based on maximizing the return power P₁. Thetransmitter is based on self-seeding with Faraday rotators.

FIG. 4A: Block diagram illustrating the physical parameters that arerelevant for mode tuning in a self-seeded network architecture.

FIG. 4B: Block diagram illustrating the physical parameters that arerelevant for mode tuning in a BLS-seeded network architecture.

FIG. 5A: Measurement data showing the relationship between opticalpowers P₁, P₂, P₄, and laser drive current I_(LD) versus temperature ina self-seeded system with an averaging reflector.

FIG. 5B: Measurement data showing the relationship between opticalpowers P₂, P₄, and laser drive current I_(LD) versus temperature in aBLS-seeded system.

FIG. 6A: Measurement data showing the effect of external modulationaveraging reflector reflectivity through a 100-GHz AWG on the powerspectral density P₂.

FIG. 6B: Measured longitudinal mode wavelengths of a Fabry-Perotgain-and-modulation chip in the presence of self-seeding using amodulation-averaging reflector.

FIG. 7A: One embodiment of a BLS-seeded transmitter optical sourceemploying GMC mode to AAWG channel tuning based on sensing the laserdrive current I_(LD).

FIG. 7B: One embodiment of a BLS-seeded transmitter optical sourceemploying GMC mode to AAWG channel tuning based on maximizing the returnpower P₂.

FIG. 8: One embodiment of the control circuit used to manage both thefirst and second control loops.

FIG. 9: Experimental illustration of the signals used in mode tuning.

SUMMARY OF THE INVENTION

This application discloses a WDM-PON transmitter that exhibitssingle-longitudinal mode operation across the operating current andtemperature range, but it uses a multi-longitudinal mode Fabry-Pérotresonator-type gain cavity. This invention is applicable to BLS-seededand self-seeded passive optical network architectures. The disclosedcolorless transmitter features Fabry-Pérot mode tuning using temperatureto select and maintain operation in one of a multiplicity oflongitudinal modes of the Fabry-Pérot cavity tuned to a specific channelfrequency of the array waveguide grating across all temperatures in theoperating temperature range.

The mode tuning is accomplished by changing the temperature of the GMCto move the mode wavelengths until the mode closest to the externallyselected wavelengths (selected by the AWG channel) is matched. From themoment when the laser source is turned on to the moment when one of themodes is locked and maintained, a certain amount of mode hopping isexpected, but once the mode is locked to the channel, the tuningalgorithm and apparatus ensure that that one mode remains active overthe entire operating temperature range of the module and the remotenode.

The advantages of this approach relative to prior art are (a) the needto minimize the front facet reflectivity on the GMC has been removed andfor some applications eliminated, and (b) there is no need forlonger-cavity lasers, in fact, any cavity length can be used whicheliminates the need for manufacturing of long lasers. The following textgives technological background.

Self-Seeding System

FIG. 4A provides the definitions of quantities used in the descriptionof the embodiments related to self-seeded network architectures. A GMC401, its associated monitor photodiode 402, and a heater 403 in thermalcontact with the GMC 401 are enclosed in an optical module 404. The GMC401 is driven by laser drive current I_(LD), while the monitorphotodiode 402 captures a portion of the light emitted from the GMC 401and converts it to monitor-photodiode current I_(PD). The heater 403maintains the GMC 401 at a temperature T_(GMC) sensed using athermocouple or a thermistor 408 in thermal contact with the GMC 401,while the ambient temperature (outside of the transceiver) is denotedwith T_(AMB). The light emitted by the GMC 401 is coupled to one of thechannels 407 on the AWG 409 using distribution fiber 406 and via opticalcoupler 405. A portion of the optical power directed from the GMC 401towards the AWG 409 is sampled using optical coupler 405 and the sampledpower is denoted with P₂, while a portion of the optical power returningfrom the AWG 409 towards the GMC 401 is sampled and the sampled power isdenoted with P₁. The optical signal emitted form the common port 413 ofthe AWG 409 is split into two portions at optical coupler 412. A portionof the optical power emitted from the common port of the AWG 409 isdirected toward the central office (P₄) and a portion is reflected at areflector 410 which is preceded by a modulation-averaging structure 411.The power returning from the modulation-averaging structure/reflector410/411 is sampled and the sampled power is denoted with P₃.

The transceiver 404 operates with two control loops at all times: Thefirst control loop adjusts the laser drive current I_(LD) so that themonitor photodiode current I_(PD) remains at a constant predeterminedvalue—this type of control loop is ubiquitous in fiber optic componentswhere the transmitter output power needs to be kept at an approximatelyconstant value independently of temperature and ageing. In embodiments,the second control loop adjusts T_(GMC) so that one select longitudinalmode of the Fabry-Pérot cavity is aligned with the passband of the AWGat all times. In one embodiment, the second control loop adjust T_(GMC)so that the drive current slope dI_(LDO)/dT and curvature d²I_(LDO)/dT²are maintained at a predetermined value and polarity, respectively. Thepreset slope is determined by experiment in which the relationshipbetween the peak in output power P₄ is related to the minimum in thedrive current I_(LDO).

BLS-Seeded System

FIG. 4B provides the definitions of quantities used in the descriptionof the embodiments related to BLS-seeded network architecture. Theoptical source 450 comprises GMC 451, its associated monitor photodiode452 and heater 453 all of which are enclosed in an optical module 454.GMC 451 is driven by drive current I_(LD), while the monitor photodiode452 captures a portion of the light emitted from the GMC 451 andconverts it to monitor-photodiode current I_(PD). The heater 453maintains the GMC 451 at temperature T_(GMC) sensed using a thermocoupleor a thermistor 464 in thermal contact with the GMC 451. The ambienttemperature is denoted with T_(AMB). The light emitted by the GMC 451 iscoupled to one of the channels 457 on the AWG 458 using distributionoptical fiber 456 and via optical coupler 455. A portion of the opticalpower directed from the GMC 451 towards the AWG 458 is sampled usingoptical coupler 455 and the sampled power is denoted with P₂, while aportion of the optical power returning from the AWG towards the GMC issampled and the sampled power is denoted with P₁. The optical signalemitted form the common port 463 of the AWG 458 is referred to as theoutput power P₄, which is directed towards the central office via trunkfiber 459. A broadband light source (BLS) 460 emits broadband light inthe wavelength range within which the GMC 451 has gain. The light fromthe BLS 460 is coupled to the trunk fiber 459 at an optical coupler 461,filtered through the AWG 458 and inserted into the GMC 451 via thedistribution fiber 456 and the optical coupler 455. The optical powerfrom fiber 462 is directed towards the central office. The directions ofoptical power and their mutual relationship can be deduced from FIG. 4Bin a straightforward manner.

The transceiver 454 operates with two control loops at all times: Thefirst control loop is adjusting the GMC drive current I_(LD) at a valuethat maintains the monitor photodiode current I_(PD) at a constantpredetermined value. The second control loop adjusts T_(GMC) so that alltimes one select longitudinal mode of the Fabry-Pérot cavity is alignedwith the passband of the AWG. In one embodiment, T_(GMC) is maintainedat a value for which I_(LDO) exhibits a minimum. In another embodiment,T_(GMC) is maintained at a value for which P₂ exhibits a minimum.

Frequency f and free-space wavelength λ are used interchangeably in thisapplication. They are related by λf=c, where c is the speed of light infree space. Unless otherwise stated, wavelength λ refers to free space(vacuum). Wavelength in the material will be denoted with λ_(m).

DETAILED DESCRIPTION

The detailed description discloses mode two tuning approaches (1 forcurrent and 2 for optical power) for each of representative colorlessWDM-PON architectures: A for self-seeding with modulation averagingreflectors (MAR system), B for self-seeding with Faraday rotators (FRsystem), and C for BLS-seeded system (BLS system)—a total of six (6)alternatives: A1, A2, B1, B2, C1, and C2.

I. Gain and Modulation Chip (GMC)

The GMC features a waveguide through which electromagnetic wavespropagate and within which they experience both gain and wherespontaneous emission is added. The GMC in this application has thestructure of a Fabry-Pérot resonator. The GMC is designed to have gainacross the entire upstream band: In other words, there is sufficientgain in the GMC to amplify each of the wavelengths used in the upstreamcommunication even though the GMC will be amplifying and communicatingonly at one of those wavelengths. If the upstream band of wavelengths iswider than what can be accomplished with a single semiconductor activeregion, two different types of gain media (and hence two differenttransmitting chains) may be used to cover the entire upstream band. Sucha system could be referred to as a two-gamut system, rather than acolorless (or single-gamut) system. The inventions disclosed may beapplied to such a system without departing from the spirit of theinvention.

A Fabry-Pérot resonator has two mirrors, one at each end of thewaveguide as is well known in the art (a front facet mirror and a backfacet mirror), and they have non-zero reflectivity for electromagneticwaves. The reflectivity of an uncoated facet of a typical semiconductorFabry-Pérot laser is around 30%, while high-reflectivity coating on afacet can increase this reflectivity to over 95%. Anti-reflectivecoatings can reduce this reflectivity below 1%, and if the front-facetalso cuts the waveguide at an angle, reflectivities below 0.01% areachievable. Generally, the waveguide may contain other reflectivefeatures (other than the two mirrors) and may feature a emissionspectrum that is more complex than that of a Fabry-Pérot resonator. Theinvention disclosed will be applicable as long as there is multiplicityof longitudinal modes available for lasing and coupling to a channel ofan external AWG.

Other tunable laser structures, such as, tunable DBR and DFB lasers havebeen used to match their wavelengths to an AWG channel, but they are allsingle-longitudinal mode devices and the tuning has to adjust thatsingle longitudinal mode to the center wavelength of the passband of theAWG channel. There are no multiple modes to choose from. Additionally,when tunable DBR and DFB lasers are used, there is no feedbackinformation on whether the wavelength of the AWG has been adjusted: thetunable lasers have own wavelength and power stabilization algorithmsand hardware. In other words, the tuning is “open loop” in that thewavelength choice is set by a certain current, temperature or phasesetting on the chip all being predetermined at manufacturing time. Thereis no information from the remote node about whether the wavelength isright.

In this invention, on the other hand (a) the optical source has multiplelongitudinal modes rather than a single mode so the tuning method firstselects one of those modes to lock to, and (b) the tuning methodmaintains real time adjustment to the wavelength specified by thechannel of the AWG because there is always information from the remotenode arriving to the transceiver: both for self-seeded and BLS-seededsystems.

It is important to note that the tuning apparatuses and method disclosedwithin are different from what is generally referred as tuning with an“intra-cavity filter”, because the AWG (serving as the filter) isexternal to the cavity being tuned. Furthermore, it is also important tonote that the tuning apparatuses and method disclosed within aredifferent from what is generally referred as temperature tuning of thegain or gain peak, because it is the channel of the AWG that determinesthe wavelength and not the gain peak of the semiconductor laser.

No matter how low the facet reflectivity, the two facets will form aresonator around the central gain region and the gain and emissionspectra will exhibit peaks (resonances) at wavelengths whereconstructive interference occurs in the resonator and valleys atwavelengths where the interference is destructive. The peaks and valleysin the spectra are commonly referred to as fringes and are approximatelyperiodic in frequency for a classical Fabry-Pérot structure. Thewavelengths of the peaks, also referred to as modes or mode frequencies,and their separation depend on the refractive index, the refractiveindex dispersion (wavelength dependence of the refractive index) in theresonator n(f) and the length of the resonator L. The mode frequenciesf_(m) and their separation Δf_(m) are given with f_(m)=mc/2 nL, where mis the mode number, and Δf_(m)=c/2 nL, respectively. These facts arewell known in the art of semiconductor laser and optical amplifiertechnology and can be found in publicly available texts, such as, L. A.Coldren and S. W. Corzine, Diode Lasers and Optical Integrated Circuits,published by Willey and Sons. Inasmuch as WDM-PON systems are generallysingle transverse mode, the Fabry-Pérot structures in this applicationare all single transverse mode devices.

II. Design of WDM-PON Source with Single Longitudinal Mode

To ensure that only one mode appears in the passband of the AWG at alltimes, one has to satisfy two conditions: (i) the longitudinal modeseparation of the Fabry-Perot resonator of the GMC has to be larger thanthe FWHM of the AWG channel passband, and (ii) one has to tune thewavelength of that selected mode to remain in the passband of the AWGfor all ambient temperatures.

We have experimentally estimated that only one mode will oscillate ifthe Full-Width Half Maximum (FHWM) of the array-waveguide channelpassband is equal to or smaller than the longitudinal mode separationΔf_(GMC) of the Fabry-Perot resonator with gain (the GMC). Thiscondition is expressed as FWHM≦Δf_(GMC), where both FWHM and Δf_(GMC)may be expressed in frequency (GHz) or in wavelength (nm). For example,we have observed experimentally that for an AWG with 100 GHz channelseparation and a passband width FWHM=0.40 nm, using a laser withΔf_(GMC)=0.53 nm never oscillates in more than one mode. On the otherhand, using a gain medium with mode separation Δf_(GMC)=0.32 nm, weobserve two mode operation at some temperatures.

Given a semiconductor cavity length, one can estimate the amount oftemperature tuning necessary to adjust the mode to a correct wavelength.The resonant modes of the GMC chip change with the optical length of thecavity. The typical drift in the mode frequencies of semiconductor chipetalon (Fabry-Perot cavity) modes is approximately β≈+0.1 nm/° C. or−12.5 GHz/° C. If the mode separation between the gain-chip modes isΔf_(GMC), then the change in chip temperature necessary to bring themode into the center of the passband is Δf_(GMC)/β, which for theexample shown above equals about 4° C. This is a relatively smalltemperature change and it is easily applied to the GMC using a heater ora thermoelectric cooler attached to it.

The GMC temperature T_(GMC) is controlled by mounting the GMC to aheater or a thermoelectric cooler. Either one can be used withoutdeparting from the invention. The thermoelectric cooler offers thepossibility to reduce the temperature below the ambient temperature, butat the expense of efficiency.

The essential advantage of the disclosed tuning principle is that thesame GMC cavity can be used for AWGs with any frequency grid smallerthan the grid for which it was designed. Namely, the tuning range isdetermined by the GMC cavity, and not by the AWG passband. For thisreason, one GMC cavity design will serve all frequency grids that arefiner than the one for which the GMC cavity was designed. Clearly, oncea gain-and-modulation device is selected and Δf_(GMC), fixed, we can usethat gain device and generate a single longitudinal-mode device with anyarray-waveguide grating that has FWHM≦Δf_(GMC). In other words, a laserthat tracks a channel of a 100 GHz AAWG with a single mode, will workequally well on a 50 GHz AAWG.

In embodiments presented below, whether and when a longitudinal mode ofthe gain medium is matched to the passband of the array-waveguidegrating will be determined by on the type of WDM-PON architecture isused: (1 & 2) self-seeding or (3) broad-light source seeding and on thedesired complexity of the transceiver: (A) sensing current or (B)sensing optical power. As will be disclosed in text below, mode matchingwill coincide with (a) a local minimum in the drive currentI_(LDO)(T_(GMC)) or emitted optical power P₂(T_(GMC)) relationships forBLS-seeding, namely, slope dI_(LDO)/dT≈0 and curvature d²I_(LDO)/dT²>0(and equivalently for emitted power P₂), (b) a local maximum in thereturn optical power P₁(T_(GMC)) relationship for self-seeding, namely,slope dP₁/dT≈0 and curvature d²P₁/dT²<0, (c) a predetermined slope andcurvature polarity in drive current I_(LDO)(T_(GMC)) relationship forself-seeding, namely, slope dI_(LDO)/dT≈fixed value and curvatured²I_(LDO)/dT² fixed polarity.

Additionally, forcing the predetermined slope and curvature ofI_(LDO)(T_(GMC)) disclosed in (c) above may also be adjusted by forcingthe I_(LDO)(T_(GMC)) to take value that is defined by a predeterminedfraction of the separation between the maximal max[I_(LDO)(T_(GMC))] andthe minimal min[I_(LDO)(T_(GMC))] value of I_(LDO)(T_(GMC)). In thisway, the selected predetermined value of I_(LDO)(T_(GMC)) is definedrelative to the rest of the I_(LDO)(T_(GMC)) relationship which can beestablished every time at the power-up of the transceiver. Furthermore,the case (c) above can be improved by dividing the slope of theI_(LDO)(T_(GMC)) by the difference between max[I_(LDO)(T_(GMC))] andmin[I_(LDO)(T_(GMC))] to obtain a relative response that is notdependent on the absolute changes in the I_(LDO)(T_(GMC)) relationship,but only on the shape.

III. The Effect of Facet Reflectivity in a Self-Seeding Architecture

The effect of non-zero front facet reflectivity is illustrated with thehelp of FIG. 6A which shows the emission spectra from a GMC biased atconstant current externally terminated with a Gaussian AAWG with FWHM=52GHz as the external mirror reflectivity is changed from near zero (a)R_(EXT)≈0 to maximum (d) R_(EXT)≈1 through two intermediate values (b)and (c). When there is no external termination (R_(EXT)≈0), the emissionfrom the gain chip shows resonant fringes with the gain-chip modeseparation Δf_(GMC)≈47 GHz and fringe depth x≈8 dB. Fringe depth X isdefined as the peak power divided by valley power. Clearly, at least oneGMC mode, possibly two can fit within the FWHM of the AAGW filter asΔf_(GMC)<Δf_(3dB). As the round-trip loss in the external cavity reduces(moving from R_(EXT)≈0 towards R_(EXT)≈1), the threshold currentreduces, and once the operating current, which was kept unchanged,becomes larger than the threshold current, the gain clamps. Thresholdgain reduces as the round-trip losses reduce. Inasmuch as the fringedepth depends on the gain-chip gain, it also reduces with the reducingexternal cavity losses. Furthermore, a reduction in gain means increasedabsorption, which in turn increases the refractive index of the cavitycausing the GMC modes to move towards longer wavelengths. Fringe depth Xis related to the product RG of the round-trip gain G=exp(2 gL) andround trip power reflectivity R=R_(F)R_(B), where R_(F) and R_(B), areoptical power reflectivities of the front and the back facet of the gainchip, respectively, via: √{square root over (RG)}=(√{square root over(x)}−1)/(√{square root over (x)}+1). The GMC measured for FIG. 6A hasunsaturated gain G≈27 dB (for the R_(EXT)≈0 curve) and if we assume thatthe back facet is reflectivity is high (R_(B)˜1), we obtain front facetreflectivity R_(F)<250 ppm. At the lowest round-trip loss (R_(EXT)≈1case) the gain compresses to G≈7 dB, which under the same assumptionsyields fringe depth x≈0.6 dB in the final emission spectra (this smallvariation is visible at the top of the R_(EXT)≈1 curve).

Following the analysis of FIG. 6A, it is clear that coating the edges ofgain chips to achieve very low facet reflectivity is beneficial andnecessary to realize extended cavity resonators. When the GMC chip isexternally seeded, the effect of the front facet reflectivity is reducedis it figures as a product between the gain and the facet reflectivity:GR_(F)R_(B). However, even though very low fringe depth can be obtained,it remains dependent on the external cavity losses, it is not stable: itwill vary depending on the specific distribution fiber length andconnector quality: every time a technician disconnects and reconnectsthe network cable, the conditions change.

FIG. 6B illustrates how longitudinal mode wavelengths 601 seen in theemitted light at port P₂ move with temperature and how the externallydefined wavelength locks the mode closest to a fixed value 602determined by the AWG channel passband peak wavelength

IV. Physical Phenomena Relied on to Tune the Modes

FIGS. 5A and 5B show the experimental evidence that gives the foundationfor the disclosed methods and apparatuses for tuning the Fabry-Perotmodes in self-seeded and BLS-seeded systems. FIG. 5A shows a measurementof optical power P₁, P₂, P₄ and laser drive current I_(LD) in aself-seeded system as shown in FIG. 4A while I_(PD) is maintainedconstant and the temperature of the GMC T_(GMC) is varied byapproximately 15° C. FIG. 5B, discussed later, shows the measurement ofoptical powers P₂, P₄ and laser drive current I_(LD) in a BLS-seededsystem while I_(PD) is maintained constant and T_(GMC) is varied. Ineither case, the I_(PD) is maintained constant by adjusting I_(LD) usinga control loop as is well known in the art of fiber-optic modules.

As I_(PD) is maintained constant, one expects the laser current I_(LD)to increase and oscillate when the temperature rises. FIGS. 5A and 5Bboth show that the drive current I_(LD) oscillates as well as increaseswith temperature. It will be useful to interpret this variation as asuperposition of two components: a component I_(LDL) that grows almostlinearly with temperature and a component I_(LDO) that oscillates intemperature: I_(LD)=I_(LDL)+I_(LDO). The almost-linear term I_(LDL)results from temperature properties of the GMC gain medium, the materialand the laser resonator design and average losses, while the oscillatingpart I_(LDO) originates from the interaction of the GMC with theexternal filter, the AWG. When we search for an extremum in the drivecurrent, we mean searching for the extremum in the oscillating partI_(LDO). When the slope dI_(LD)L/dT (linear component) is small incomparison with the slope in dI_(LDO)/dT (oscillating component), theminima in the measured I_(LD) and I_(LDO) are approximately equal andhence the minimum in the measured I_(LD) can be used in place of theminimum in I_(LDO). However, when this is not the case, one should firstestimate the slope of I_(LDL) versus temperature and subtract it frommeasured I_(LD) to get an estimate of I_(LDO) and then determine theminimum or maximum on the approximate versions of I_(LDO). Inasmuch asthe slope dI_(LDL)/dT is a property of the material and design, it doesnot vary much between different GMC units. For this reason, anapproximate value of the slope can be determined at manufacturing timefrom a collection of GMC units and then that same number can be used forall modules. Removing just the first-order linear term from I_(LD)significantly improves the accuracy of the extrema search or specificslope in I_(LDO). Finally, when we refer to a minimum in the drivecurrent I_(LD) in the text, we mean a minimum in I_(LDO), theoscillating component of I_(LD) and expect that the I_(LDL) term hasbeen subtracted from the measured I_(LD).

Continuing with the general behavior of graphs shown in FIGS. 5A and 5B,it is visible that as the external (AWG passband) wavelength scans themode spectra, it sometimes coincides with a longitudinal mode of theFabry-Perot cavity in which case due to the constructive interference,the emission and light amplification is higher for lower current. On theother hand, when the passband wavelength produces destructiveinterference in the cavity, the emission and amplification is lower forthe same current.

IV.1 Self-Seeded System Measurements

The graphs in FIG. 5A show that as temperature increases, the opticalpowers P₁, P₂, and P₄, all oscillate as the externally definedAWG-channel wavelength scans though the Fabry-Pérot modes: theFabry-Pérot modes move almost linearly with the temperature, while theAWG-channel wavelength is fixed. The laser drive current I_(LD)on-the-average increases with the temperature, but also oscillates. Asnoted before, we are interested in the oscillatory portion of the laserdrive current. The challenge for the tuning hardware and method is tomaximize the output power P₄, while using only the information availableto the client end, namely, in the transceiver, next to the GMC. Inasmuchas the distribution fiber 406 can be several kilometers long and thatthe optical coupler 405 may be integrated with the transceiver 404, themeasured quantities that may be used as feedback for tuning are P₁, P₂or I_(LD).

The graphs in FIG. 5A show that P₄ is proportional to P₁ (peaks 510 and511 are at the same temperature) and that P₂ is approximatelyproportional to I_(LD), but that local minima 512 in I_(LD) (or morecorrectly, the minima in the oscillating portion of I_(LD)) do notexactly correspond to the maxima 510 in P₄ as one would expect. They areoffset by a small temperature ΔT_(OFFSET)=min(I_(LD))−max(P₄)−1° C.Furthermore, neither of the optical signals intensities P₁, P₂, P₄ aresinusoidal with temperature as one would expect. The offset and theasymmetry are caused by a combination of two phenomena: (a) morespontaneous emission is present in the light captured by the monitorphotodiode (I_(PD)) and in the optical power P₂, than in the lightfiltered through the AWG (P₁ and P₄), so the optical power measured atP₂ and contributed by I_(LD), comprises of two components that varydifferently with the wavelength and intensity, and (b) the seedingintensity changes the refractive index in the cavity adding anexternal-filter influenced wavelength shift. The latter effect wasdiscussed in connection with FIG. 6A. Increased temperature shiftslongitudinal modes towards longer wavelengths, but the loss-dependentmode pulling effect illustrated in FIG. 6A adds a non-linearity: modesare pulled towards longer wavelengths (same direction as increasingtemperature) when a mode wavelength is moving towards the centerwavelength of the AWG channel passband (from either wavelengthdirection). This means that mode wavelength drift with temperature islarger on one side of the AWG channel than on the other. When combinedwith oscillating mode intensity and oscillating spontaneous emissionintensity with temperature, these two phenomena (temperature increaseand external loss change) cause the asymmetry in the response curvesshown in FIG. 5A.

IV.2. BLS-Seeded System Measurements

The graphs in FIG. 5B show that as temperature increases, the opticalpowers P₂ and P₄, all oscillate as the externally defined AWG-channelwavelength scans through the Fabry-Pérot modes: the Fabry-Pérot modesmove almost linearly with the temperature. The laser drive currentI_(LD) on the average increases with the temperature, but alsooscillates. As noted before, we are interested in the oscillatoryportion I_(LDO) of the laser drive current I_(LD). The challenge for thetuning hardware and method is to maximize the output power P₄, whileusing only the information available to the client end, namely, in thetransceiver, next to the GMC. Inasmuch as the distribution fiber 456 canbe several kilometers long, the measured quantities that may be used asfeedback for tuning are P₁, P₂ or I_(LD).

The graphs in FIG. 5B show that P₄ is proportional to P₂, and that P₂ isapproximately proportional to the oscillatory portion of I_(LD). Theoutput power versus temperature P₄ is approximately sinusoidal. Here itis clear that to adjust the output power to a maximum 522, one needs toeither maintain the temperature of the GMC at a value that gives aminimum of the I_(LDO) 521 or a minimum in P₂. It appearscounter-intuitive that higher output power P₄ should correlate withlower GMC output power P₂. The reason for this is that extra spontaneousemission power that is present in the P₂ emission, but not in P₄.Spontaneous emission is proportional to the drive current I_(LD).

V. Network Component Embodiments

In one embodiment, the GMC is a reflective semiconductor opticalamplifier (RSOA), which includes an anti-reflective coating and mayinclude a curved waveguide on the front facet and a high-reflectivitymirror on the back facet. The front facet may exhibit reflectivitiesbelow 100 ppm. The back facet is typically designed for transmissionthat provides sufficient amount of light to the monitor photodiode.

In another embodiment, the GMC is a coated Fabry-Perot laser (CFP),which includes an anti-reflective coating on the front facet and ahigh-reflectivity mirror on the back facet. The front facet may exhibitreflectivities below 2000 ppm. The back facet is typically designed fortransmission that provides sufficient amount of light to the monitorphotodiode and it may not be coated at all.

In another embodiment, the GMC is an uncoated Fabry-Perot (UFP) laser.The front facet may exhibit reflectivities around 30% as is well knownin the art. The light emanating from the back facet is directed towardsa monitor photodiode.

In all GMC embodiments, it is essential that devices exhibit amultiplicity of longitudinal modes. The RSOA, CFP, and UFP arestructurally similar and they all are inherently multi-longitudinal modedevices.

In all GMC embodiments, the monitor photodiode may be mounted to capturethe light emitted from the back facet or to capture a portion of lightemitted from the front facet. In this application, the front facet ofthe GMC is the output facet or the one where light used in the opticalcommunications exits. The figures in this application all show anddisclose architectures where the monitor photodiode is mounted on theback facet, but it is clear that the embodiments apply to thearrangements where the monitor photodiode is mounted on the front facet.

In this application, monitor photodiode has a slow response;sufficiently slow that it does not follow the data modulation, itmeasures the time-averaged light intensity. High-speed photodiode hassufficient bandwidth to capture the amplitude modulation of the datastream. Such a high-speed photodiode also captures the time-averagedintensity.

The length of distribution fibers in the embodiments range from tens ofmeters to tens of kilometers. The frequency (wavelength) band containingthe WDM channels used for downstream transmission are referred to thedownstream band, and similarly for the upstream band. In the followingdescription of the invention, C-band is used for upstream, and L-bandfor downstream traffic, noting that any combination of bands orfractions of a band can be used with this invention without departingfrom the spirit of the invention.

The array-waveguide grating (AWG) is an opticalmultiplexer/demultiplexer which demultiplexer wavelength-divisionmultiplexed optical signals entering a common port into M signals at Mdifferent distribution ports with M different wavelengths. For anyoptical signal arriving to any of the distribution ports, the AWG iseffectively a bandpass optical filter centered at a center wavelengthand a bandpass bandwidth expressed. Both the center wavelength andbandwidth may be expressed in terms of wavelength or frequency withoutloss of generality. For the purposes of this application, a passband isa wavelength range (or frequency range) for which the transmission fromthe distribution port to the common port is greater than one half of themaximum transmission at the center of the passband or the transmissionat the center wavelength. The terms used for characterize the bandwidthof such bandpass filters are 3-dB bandwidth orfull-width-of-half-maximum (FWHM). The AWG is typically cyclical so thatwavelength-division multiplexed signals arriving at the common port ofthe AWG with wavelength separation equal to the free-spectral range(FSR_(AWG)) of the AWG are mapped into the same of the M channels on thedistribution ports as is well known in the art. The M channelwavelengths and wavelength separation between the adjacent channels onthe AWG are specified by the appropriate standard for WDMcommunications. The wavelength separation is typically expressed infrequency separation; typical standard ITU channel grids are presentlyΔf_(AWG)=200 GHz, 100 GHz, 50 GHz, or 25 GHz grid, which means that thefrequency separation between adjacent channels (center to center) isapproximately equal to Δf_(AWG). Athermal AWG (AAWG) means that it hascompensated thermal drift of the channel frequencies. The type of AAWGof interest in this invention is a cyclical AAWG covering C and L bands,so that these two bands can be used for separate upstream and downstreamsignals. However, the cyclical property is not required for practicingthe invention. In other words, channel 7 in the C-band, for example,used for upstream data transfer has it dual in the L-band: channel 7 inthe L-band is used for downstream signaling, and consequentlydistribution port 7 passes channel 7 in the C-band and channel 7 in theL-band. The channel separation and numbering used in this example are100 GHz and fc(k)=196.2−0.1 ·k [THz] and f_(L)(k)=191.2−0.1·k [THz], forC and L bands, respectively. The AWG 202 is athermal in that it providestemperature sensitivity that is low enough to satisfy ITUrecommendations on the channel frequencies without the requirement forcooling or heating—it remains passive. Each passband is specified by thefilter shape which may Gaussian or flat, and the bandwidth to a certaindegree characterized with the 1 dB and 3 dB bandwidths. For the purposesof this application, we shall use the term full-width at half maximum(FWHM) for the 3-dB bandwidth the AWG 202 passband to characterize theAWG passband.

A modulation averaging reflector comprises a mirror and a modulationaveraging structure. Modulation averaging structures can be implementedin fiber or planar lightwave circuit technology and are described in USpatent applications 20140029083 and 20140010544, and U.S. Pat. No.8,559,775.

A reflector assembly is an optical component comprising one reflectionport and one transmission port. Optical signals entering the reflectionport are partially transmitted and exit at the transmission port, whilea portion of the optical signal is reflected back and exit at thereflection port. For the purposes of this application, a reflectorassembly may include a modulation-averaging structure arranged with amirror and at least one optical coupler, in which case it is referred toas a reflector assembly with modulation averaging. A reflector assemblymay include a Faraday rotator and mirror (effectively a Faraday rotatingmirror) in which case it is referred to as a reflector assembly with aFaraday rotator. Finally, a reflector assembly may include both aFaraday rotator and a modulation averaging structure, all depending onthe specific design and requirements of the network.

VI. Method Embodiments

In the above text it was noted that the first control loop (varyinglaser current to maintain constant monitor photodetector output) iscommon to fiber-optic modules. The second control loop is unique andinnovative and the following embodiments are based on three differentimplementations of the second control loop.

In one embodiment, the second control loop adjusts the temperatureT_(GMC) to a value where the laser drive current I_(LD) is fixed withpredetermined slope and curvature. This embodiment of the transmitterand control loop is applicable to self-seeded systems. When broadbandlight sources (BLS) seeded systems are used, the I_(LDO) is minimized.

In another embodiment, the second control loop adjusts the temperatureT_(GMC) to a value where the emitted power P₂ is minimized. Thisembodiment of the transmitter and control loop applied to BLS-seededsystems.

In yet another embodiment, the second control loop adjusts thetemperature T_(GMC) to a value where the emitted power P₁ is maximized.This embodiment of the transmitter and control loop applied toself-seeded systems.

VII. Self-Seeded System Embodiments with Modulation Averaging

In one embodiment, illustrated in FIG. 2A, a WDM optical transmittersection 200 of an WDM-PON system comprises a remote node 209 and atleast one optical transceiver 230 connected to remote node 209 using adistribution fiber 220. To operate the WDM-PON source, at least onetransceiver is required, but FIG. 2A illustrates three opticaltransceivers 230, 222, and 224 connected to the remote node 209, eachtransceiver using its own distribution fiber 220, 221, and 223,respectively. FIG. 2B illustrates three optical transceivers 260, 222,and 224 connected to the remote node 309, each transceiver using its owndistribution fiber 220, 221, and 223. The distance between the remotenode 209 and any of the transceivers 230, 222, and 224 is determined bythe available link budget, desired line rate, and dispersion penalty asis well known in the art.

The remote node 209 comprises an array waveguide grating (AWG) 202 withat least one common port and M distribution ports 225, an optical outputcoupler 203, a third mirror 205 and a modulation averaging structure 204which together with the third mirror 205 forms a modulation-averagingreflector. Output coupler 203, modulation averaging structure 204, andthe mirror 205 constitute the reflector assembly of this embodiment. TheAWG 202 is an optical multiplexer/demultiplexer which demultiplexeswavelength-division multiplexed optical signals entering common portinto M signals at M different distribution ports 225 with M differentwavelengths.

The third mirror 205 is a high-reflectivity mirror disposed at the endof a fiber. Such fiber mirrors are commercially available from a numberof manufacturers. The reflectivity spectrum of the mirror has highreflection coefficient, greater than 90% over the entire upstreamfrequency (wavelength) band. The modulation averaging structure 204 is apassive optical component that averages amplitude modulated opticalsignals, and may also depolarize the optical signals.

In one embodiment, the transceiver 230 comprises GMC, duplexer, receiverchain and transmitting chain, including the heater are enclosed into asingle housing, such as an SFP or XFP module, but may also be onlypartially packaged without departing from the spirit of the invention.

There are two transceiver embodiments that feature mode tuning. In FIG.2A the tuning is done using I_(LD) and in FIG. 2B using returned opticalpower P₁. The remote node and trunk fiber are numbered identically inFIGS. 2A and 2B to stress that either one of the transceivers shown inFIG. 2A and FIG. 2B may be used with the same arrangement in the remotenode.

VII.1. Sensing Laser Drive Current (A1)

Architecture 200 uses at least one transceiver 230 which comprises anoptical port 231 which may be connectorized, electrical ports 236, 247,and 248, a duplexer 232 optically coupled to the optical port 231, GMC240 which includes front facet mirror 241 and back-facet mirror 242, aback-facet monitor photodiode 243, a laser driver chip “LD” 244, ahigh-speed photodetector 233, a transimpedance amplifier 234, a receiverchip “RX” 235, a heater stage “TEC/HT” 249 in thermal contact with GMC240, and a controller “CNT” 245. The electrical ports comprise receiveroutput (downstream data output) 236 which may be differential,transmitter data input (upstream data input) 247 which also may bedifferential, and module diagnostics port 248 coupled to the controller245 which may send traffic in either direction. The controller 245monitors the temperature using a thermocouple or thermistor via line252; the temperature sensor is denoted with the dot at the end of theline 252. The controller 245 controls the temperature of the GMC 240 byenergizing the heater or thermoelectric cooler 249. The controller 245furthermore communicates with the laser driver chip 244 via line 246: itsets the desired monitor photodiode current I_(PD) (effectively GMC 240output power) and sends control signals to the laser driver 244, suchas, laser off and on, and receives the information on the magnitude ofthe drive current I_(LD) and diagnostic signals, such as, for example,laser fault, over heating, and so on.

The transceiver is operatively configured to receive and emit opticalsignals through the optical fiber port 231 and electrical data andcontrol signals through electrical ports 236, 247, and 248. Opticalsignals with wavelengths in the downstream wavelength range are incidentthrough the optical fiber power 231 are routed by the duplexer 232 tothe high-speed photodetector 233 where the optical intensity isconverted to electrical current and then converted to voltage at thetrans-impedance amplifier 234. The analog signal at the exit of thetrans-impedance amplifier 234 is amplified and further processed in thereceiver 235 before it is routed to the electrical ports 236 associatedwith the transceiver electrical output. The high-speed photodetector233, transimpedance-amplifier 234, and the receiver 235 constitute thereceiving chain of transmitter 230.

The GMC 240 is driven directly using a laser driver 244 which brings DCbias (operating point) and the AC modulation according to the data input247 as is well known in the art. The current output from the monitorphotodiode 243 coupled to the GMC 240 is fed to the laser driver 244 asinput to the analog feedback loop that maintains the laser outputconstant over time and temperature.

The longitudinal mode separation Δf_(GMC) of the GMC 240 is designed toensure that only one mode ever appears within the passband of thechannel of the external AWG, hence Δf_(GMC)≧FWHM.

In one embodiment, the mode tuning in a WDM-PON transmitter architectureshown in FIG. 2A comprises measuring the photodetector current I_(PD)and simultaneously adjusting the GMC drive current I_(LD) to maintainthe I_(PD) at a preset value. The controller further periodicallydithers the heater drive power, measures a periodic oscillation in GMCtemperature and the resulting periodic oscillation in the GMC drivecurrent (I_(LD)). The controller is operatively configured to use thetime evolution of GMC drive current I_(LD) and the temperature T_(GMC)to maintain the average GMC temperature T _(GMC) at a value for whichthe slope dI_(LDO)/dT is fixed to a non-positive number and thecurvature is negative (see the slope of I_(LD) where P₄ is at a maximumin FIG. 5A). An alternative is setting the I_(LD) at a preset fractionof the value between its min(I_(LD)) and max(I_(LD)). In this way, onelongitudinal mode of the GMC will remain within the passband of thearray-waveguide array regardless of the ambient temperature. The case ofthe transceiver 230 is assumed to be at an ambient temperature T_(A)which is generally different from the GMC 240 temperature T_(GMC).

VII.2. Sensing Return Optical Power (A2)

Architecture 201 uses at least one transceiver 260 which comprises anoptical port 261 which may be connectorized, electrical ports 266, 277,and 278, a duplexer 262 optically coupled to the optical port 261, again and modulation chip (GMC) 270 which includes front facet mirror 271and back-facet mirror 272, a back-facet monitor photodiode 273, a laserdriver chip “LD” 274, a high-speed photodetector 263, a transimpedanceamplifier 264, a receiver chip “RX” 265, a heater stage “TEC/HT” 279 inthermal contact with GMC 270, and a controller “CNT” 275. The electricalports comprise receiver output (downstream data output) 266 which may bedifferential, transmitter data input (upstream data input) 277, andmodule diagnostics port 278 coupled to the controller 275 which may sendtraffic in either direction. The controller 275 monitors the temperatureusing a thermocouple or thermistor via line 292; the temperature sensoris denoted with the dot at the end of the line 292. The controller 275controls the temperature of the GMC 260 by energizing the heater orthermoelectric cooler 279. The controller 275 furthermore communicateswith the laser driver chip 274 via line 276: it sets the desired monitorphotodiode 273 current I_(PD) or GMC 240 output power via monitorphotodiode 282 and sends control signals to the laser driver, such as,laser off and on, and receives the information on the magnitude of thedrive current I_(LD) and diagnostic signals, such as, for example, laserfault, over heating, and so on.

The transceiver is operatively configured to receive and emit opticalsignals through the optical fiber port 261 and electrical data andcontrol signals through electrical ports 266, 277, and 278. Opticalsignals with wavelengths in the downstream wavelength range are incidentthrough the optical fiber power 261 are routed by the duplexer 262 tothe high-speed photodetector 263 where the optical intensity isconverted to electrical current and then converted to voltage at thetrans-impedance amplifier 264. The analog signal at the exit of thetrans-impedance amplifier 264 is amplified and further processed in thereceiver 265 before it is routed to the electrical ports 266 associatedwith the transceiver electrical output. The high-speed photodetector263, transimpedance-amplifier 264, and the receiver 265 constitute thereceiving chain of transmitter 260.

The GMC 270 is driven directly using a laser driver 274 which brings DCbias (operating point) and the AC modulation according to the data input277. The current output from the monitor photodiode 273 coupled to theGMC 270 is fed to the laser driver 274 as input to an analog feedbackloop within the laser driver 274 that maintains the laser outputconstant over time and temperature.

The longitudinal mode separation Δf_(GMC) of the GMC 270 is designed toensure that only one mode ever appears within the passband of thechannel of the external AWG, namely, Δf_(GMC)≧FWHM.

The transceiver 260 further comprises a beam splitter 281 andincident-light monitor photodiode 282 which is positioned between thefront facet 271 of the GMC 270 and the optical port 261. Thebeam-splitter 281 couples a fraction of the optical power incident ontothe front facet to the incident-monitor photodiode 282. The output fromthe incident-light monitor photodiode 282 is fed to the controller 275.

In one embodiment, the mode tuning in a WDM-PON transmitter architectureshown in FIG. 2B comprises measuring the photodetector current I_(PD)and simultaneously adjusting the GMC drive current to maintain theI_(PD) at a preset value. The controller further periodically dithersthe heater drive power, measures a periodic oscillation in GMCtemperature and the resulting periodic oscillation in the returned powerP₁. The controller is operatively configured to use the time evolutionof returned power P₁ and the temperature T_(GMC) to maintain the averageGMC temperature T _(GMC) at a value for which the returned power P₁ ismaximized. In this way, one longitudinal mode of the GMC will remainwithin the passband of the array-waveguide array regardless of theambient temperature. The case of the transceiver 260 is assumed to be atan ambient temperature T_(A) which is generally different from the GMC270 temperature T_(GMC).

VIII. Self-Seeded System Embodiments with Faraday Rotation

In one embodiment, illustrated in FIG. 3A, a WDM optical transmittersection 300 of a WDM-PON system comprises a remote node 309 and at leastone optical transceiver 330 connected to remote node 309 using adistribution fiber 320. To operate the WDM-PON source, at least onetransceiver is required, but FIG. 3A illustrates three opticaltransceivers 330, 322, and 324 connected to the remote node 309, eachtransceiver using its own distribution fiber 320, 321, and 323,respectively. FIG. 3B illustrates three optical transceivers 360, 322,and 324 connected to the remote node 309, each transceiver using its owndistribution fiber 320, 321, and 323. The distance between the remotenode 309 and any of the transceivers 330, 322, and 324 is determined bythe available link budget, desired line rate, and dispersion penalty asis well known in the art.

The remote node 309 comprises an array waveguide grating (AWG) 302 withat least one common port and M distribution ports 325, a 45-degreeFaraday rotator 303, a third mirror 305. Faraday rotator 303 and mirror305 constitute the reflector assembly of this embodiment. In anotherembodiment, the reflector assembly includes an optical coupler, Faradayrotator 303 and mirror 305 (not shown). The AWG 302 is an opticalmultiplexer/demultiplexer which demultiplexes wavelength-divisionmultiplexed optical signals entering common port into M signals at Mdifferent distribution ports 325 with M different wavelengths.

The Faraday Rotating Mirror comprising third mirror 305 and the Faradayrotator 303 is disposed at the end of a fiber. Such fiber mirrors arecommercially available from a number of manufacturers. The reflectivityspectrum of the third mirror has high reflection coefficient, greaterthan 90% over the entire upstream frequency (wavelength) band.

In one embodiment, the transceiver 330 comprises GMC, duplexer, receiverchain and transmitting chain, including the heater are enclosed into asingle housing, such as an SFP or XFP module, but may also be onlypartially packaged without departing from the spirit of the invention.

There are two transceiver embodiments that feature mode tuning. In FIG.3A the tuning is done using I_(LD) and in FIG. 3B using returned opticalpower P₁. The remote node and trunk fiber are numbered identically inFIGS. 3A and 3B to stress that either one of the transceivers shown inFIG. 3A and FIG. 3B may be used with the same arrangement in the remotenode.

VIII.1 Sensing Laser Drive Current (B1)

Architecture 300 uses at least one transceiver 330 which comprises anoptical port 331 which may be connectorized, electrical ports 336, 347,and 348, a duplexer 332 optically coupled to the optical port 331, GMC340 which includes front facet mirror 341 and back-facet mirror 342, aback-facet monitor photodiode 343, a laser driver chip “LD” 344, ahigh-speed photodetector 333, a transimpedance amplifier 334, a receiverchip “RX” 335, a heater stage “TEC/HT” 349 in thermal contact with GMC340, and a controller “CNT” 345. A Faraday rotator 351 is disposedbetween the duplexer 332 and the GMC 340. The electrical ports comprisereceiver output (downstream data output) 336 which may be differential,transmitter data input (upstream data input) 347 which also may bedifferential, and module diagnostics port 348 coupled to the controller345 which may send traffic in either direction. The controller 345monitors the temperature using a thermocouple or thermistor via line352; the temperature sensor is denoted with the dot at the end of theline 352. The controller 345 controls the temperature of the GMC 340 byenergizing the heater or thermoelectric cooler 349. The controller 345communicates with the laser driver chip 344 via line 246: it sets thedesired monitor photodiode 343 current I_(PD) (effectively GMC 340output power) and sends control signals to the laser driver 344, suchas, laser off and on, and receives the information on the magnitude ofthe drive current I_(LD) and diagnostic signals, such as, for example,laser fault, over heating, and so on.

The transceiver 330 is operatively configured to receive and emitoptical signals through the optical fiber port 331 and electrical dataand control signals through electrical ports 336, 347, and 348. Opticalsignals with wavelengths in the downstream wavelength range are incidentthrough the optical fiber power 331 are routed by the duplexer 332 tothe high-speed photodetector 333 where the optical intensity isconverted to electrical current and then converted to voltage at thetrans-impedance amplifier 334. The analog signal at the exit of thetrans-impedance amplifier 334 is amplified and further processed in thereceiver 335 before it is routed to the electrical ports 336 associatedwith the transceiver electrical output. The photodetector 333,transimpedance-amplifier 334, and the receiver 335 constitute thereceiving chain of transmitter 330.

The GMC 340 is driven directly using a laser driver 344 which brings DCbias (operating point) and the AC modulation according to the data input347. The current output from the monitor photodiode 343 coupled to theGMC 340 is fed to the laser driver 344 as input to the analog feedbackloop that maintains the laser output constant over time and temperature.

The longitudinal mode separation Δf_(GMC) of the GMC 340 is designed toensure that only one mode ever appears within the passband of thechannel of the external AWG, namely, Δf_(GMC)≧FWHM.

In one embodiment, the mode tuning in a WDM-PON transmitter architectureshown in FIG. 3A comprises measuring the monitor photodetector 343current I_(PD) and simultaneously adjusting the GMC 340 drive currentI_(LD) to maintain the I_(PD) at a preset value. The controller 345further periodically dithers the heater 349 drive power, measures aperiodic oscillation in GMC temperature 352 and the resulting periodicoscillation in the GMC drive current (I_(LD)). The controller 345 isoperatively configured to use the time evolution of GMC drive currentI_(LD) and the temperature T_(GMC) to maintain the average GMCtemperature T _(GMC) at a value for which the slope dI_(LDO)/dT is fixedto a non-positive value and negative curvature d²I_(LDO)/dT²<0 (seeshape of I_(LD) where P₄ is maximal in FIG. 5A). An alternative issetting the I_(LD) is at a preset fraction of the value between itsmin(I_(LD)) and max(I_(LD)). In this way, one longitudinal mode of theGMC will remain within the passband of the array-waveguide arrayregardless of the ambient temperature. The case of the transceiver 330is assumed to be at an ambient temperature T_(A) which is generallydifferent from the GMC 340 temperature T_(GMC).

VIII.2 Sensing Return Optical Power (B2)

Architecture 301 uses at least one transceiver 360 which comprises anoptical port 361 which may be connectorized, electrical ports 366, 377,and 378, a duplexer 362 optically coupled to the optical port 361, again and modulation chip (GMC) 370 which includes front facet mirror 371and back-facet mirror 372, a back-facet monitor photodiode 373, a laserdriver chip “LD” 374, a high-speed photodetector 363, a transimpedanceamplifier 364, a receiver chip “RX” 365, a heater stage “TEC/HT” 379 inthermal contact with GMC 370, and a controller “CNT” 375. The electricalports comprise receiver output (downstream data output) 366 which may bedifferential, transmitter data input (upstream data input) 377, andmodule diagnostics port 378 coupled to the controller 375 which may sendtraffic in either direction. The controller 375 monitors the temperatureusing a thermocouple or thermistor via line 392; the temperature sensoris denoted with the dot at the end of the line 392. The controller 375controls the temperature of the GMC 360 by energizing the heater orthermoelectric cooler 379. The controller 375 furthermore communicateswith the laser driver chip 374 via line 376: it sets the desired monitorphotodiode 373 current I_(PD) or GMC output power and sends controlsignals to the laser driver 374, such as, laser off and on, and receivesthe information on the magnitude of the drive current I_(LD) anddiagnostic signals, such as, for example, laser fault, over heating, andso on.

The transceiver is operatively configured to receive and emit opticalsignals through the optical fiber port 361 and electrical data andcontrol signals through electrical ports 366, 377, and 378. Opticalsignals with wavelengths in the downstream wavelength range are incidentthrough the optical fiber port 361 are routed by the duplexer 362 to thehigh-speed photodetector 363 where the optical intensity is converted toelectrical current and then converted to voltage at the trans-impedanceamplifier 364. The analog signal at the exit of the trans-impedanceamplifier 364 is amplified and further processed in the receiver 365before it is routed to the electrical ports 366 associated with thetransceiver electrical output. The high-speed photodetector 363,transimpedance-amplifier 364, and the receiver 365 constitute thereceiving chain of transmitter 360.

The GMC 370 is driven directly using a laser driver 374 which brings DCbias (operating point) and the AC modulation according to the data input377. The current output from the monitor photodiode 373 coupled to theGMC 370 is fed to the laser driver 374 as input to the analog feedbackloop that maintains the laser output constant over time and temperature.

The longitudinal mode separation Δf_(GMC) of the GMC 370 is designed toensure that only one mode ever appears within the passband of thechannel of the external AWG, namely: Δf_(GMC)≧FWHM.

The transceiver 360 further comprises a beam splitter 381 andincident-monitor photodiode 382 which is positioned between the frontfacet 371 of the GMC 370 and the optical port 361. The beam-splitter 381couples a fraction of the optical power P₁ incident onto the front facetto the incident-monitor photodiode 382. The output from theincident-monitor photodiode 382 is fed to the controller 375.

In one embodiment, the mode tuning in a WDM-PON transmitter architectureshown in FIG. 3B comprises measuring the photodetector current I_(PD)and simultaneously adjusting the GMC drive current to maintain theI_(PD) at a preset value. The controller further periodically dithersthe heater drive power, measures a periodic oscillation in GMCtemperature and the resulting periodic oscillation in the returned powerP₁. The controller is operatively configured to use the time evolutionof returned power P₁ and the temperature T_(GMC) to maintain the averageGMC temperature T _(GMC) at a value for which the returned power P₁ ismaximized. In this way, one longitudinal mode of the GMC will remainwithin the passband of the array-waveguide array regardless of theambient temperature. The case of the transceiver 360 is assumed to be atan ambient temperature T_(A) which is generally different from the GMC370 temperature T_(GMC).

IX. BLS-Seeded System Embodiments

In one embodiment, illustrated in FIG. 7A, a WDM optical transmittersection 700 of a WDM-PON system comprises a remote node 709 and at leastone optical transceiver 730 connected to remote node 709 using adistribution fiber 720. To operate the WDM-PON source, at least onetransceiver is required, but FIG. 7A illustrate three opticaltransceivers 730, 722, and 724 connected to the remote node 709, eachtransceiver using its own distribution fiber 720, 721, and 723,respectively. FIG. 7B illustrates three optical transceivers 760, 722,and 724 connected to the remote node 709, each transceiver using its owndistribution fiber 720, 721, and 723. The distance between the remotenode 709 and any of the transceivers 730 or 760, 722, and 724 isdetermined by the available link budget, desired line rate, anddispersion penalty as is well known in the art.

The remote node 709 comprises an array waveguide grating (AWG) 702 withat least one common port 712 and M distribution ports 725. The AWG 702is an optical multiplexer/demultiplexer which demultiplexeswavelength-division multiplexed optical signals entering common port 712into M signals at M different distribution ports 725 with M differentwavelengths.

The AWG common port 712 is coupled to trunk fiber 711 which is connectedto the central office. A broadband light source 780 (BLS) is coupled totrunk fiber 711 via optical coupler 710. The BLS is shown on the end oftrunk fiber 711 distal from the common port 712, while it may also belocated on the proximal end depending on whether transceiver 730 is onthe client side or the central office side. The light emitted from theBLS is directed to the AWG 702, spectrally sliced, and via thedistribution fiber 720 seeds the transceiver 730.

In one embodiment, the transceiver 730 comprises GMC, duplexer, receiverchain and transmitting chain, including the heater are enclosed into asingle housing, such as an SFP or XFP module, but may also be onlypartially packaged without departing from the spirit of the invention.

There are two transceiver embodiments that feature mode tuning. In FIG.7A the tuning is done using I_(LD) and in FIG. 7B using emitted opticalpower P₂. The remote node and trunk fiber are numbered identically inFIGS. 7A and 7B to stress that either one of the transceivers shown inFIG. 7A and FIG. 7B may be used with the same arrangement in the remotenode and central office.

VII.1. Sensing Laser Drive Current (C1)

Architecture 700 uses at least one transceiver 730 which comprises anoptical port 745 which may be connectorized, electrical ports 746, 747,and 748, a duplexer 733 optically coupled to the optical port 745, GMC735 which includes front facet mirror 742 and back-facet mirror 741, aback-facet monitor photodiode 738, a laser driver chip “LD” 737, ahigh-speed photodetector 732, a transimpedance amplifier 731, a receiverchip “RX” 736, a heater stage “TEC/HT” 743 in thermal contact with GMC735, and a controller “CNT” 740. The electrical ports comprise receiveroutput (downstream data output) 746 which may be differential,transmitter data input (upstream data input) 747 which also may bedifferential, and module diagnostics port 748 coupled to the controller740 which may send traffic in either direction. The controller 740monitors the temperature using a thermocouple or thermistor via line752; the temperature sensor is denoted with the dot at the end of theline 752. The controller 740 controls the temperature of the GMC 735 byenergizing the heater or thermoelectric cooler 743. The controller 740communicates with the laser driver chip 737 via line 751: it sets thedesired monitor photodiode current I_(PD) or GMC output power and sendscontrol signals to the laser driver, such as, laser off and on, andreceives the information on the magnitude of the drive current I_(LD)and diagnostic signals, such as, for example, laser fault, over heating,and so on.

The transceiver is operatively configured to receive and emit opticalsignals through the optical fiber port 745 and electrical data andcontrol signals through electrical ports 746, 747, and 748. Opticalsignals with wavelengths in the downstream wavelength range are incidentthrough the optical port 745 are routed by the duplexer 733 to thehigh-speed photodetector 732 where the optical intensity is converted toelectrical current and then converted to voltage at the trans-impedanceamplifier 731. The analog signal at the exit of the trans-impedanceamplifier 731 is amplified and further processed in the receiver 736before it is routed to the electrical ports 746 associated with thetransceiver electrical output. The photodetector 732,transimpedance-amplifier 731, and the receiver 736 constitute thereceiving chain of transmitter 730.

The GMC 735 is driven directly using a laser driver 737 which brings DCbias (operating point) and the AC modulation according to the data input747 as is well known in the art. The current output from the monitorphotodiode 738 coupled to the GMC is fed to the laser driver 737 asinput to the analog feedback loop that maintains the laser outputconstant over time and temperature.

The longitudinal mode separation Δf_(GMC) of the GMC 735 is designed toensure that only one mode ever appears within the passband of thechannel of the external AWG, namely, Δf_(GMC)≧FWHM.

In one embodiment, the mode tuning in a WDM-PON transmitter architectureshown in FIG. 7A comprises measuring the monitor photodetector currentI_(PD) and simultaneously adjusting the GMC drive current I_(LD) tomaintain the I_(PD) at a preset value. The controller 740 furtherperiodically dithers the heater 743 drive power, measures a periodicoscillation in GMC temperature and the resulting periodic oscillation inthe GMC drive current (I_(LD)). The controller 740 is operativelyconfigured to use the time evolution of GMC drive current I_(LD) and thetemperature T_(GMC) to maintain the average GMC temperature T _(GMC) ata value for which the slope dI_(LDO)/dT is fixed zero and the curvatureis positive (see FIG. 5B for clarification). The case of the transceiver730 is assumed to be at an ambient temperature T_(A) which is generallydifferent from the GMC 735 temperature T_(GMC).

In one embodiment, the monitoring of the GMC emissied light is doneusing a monitor photodiode mounted so that it captures a portion of theoptical power emitted from the front facet (rather than from the backfacet). In this case, the first control loop uses the front-facetmonitor diode output and the second control loop searches for theminimum in the GMC operating current as disclosed above.

VII.2. Sensing Emitted Optical Power (C2)

Architecture 701 uses at least one transceiver 760 which comprises anoptical port 761 which may be connectorized, electrical ports 766, 777,and 778, a duplexer 762 optically coupled to the optical port 761, again and modulation chip (GMC) 770 which includes front facet mirror 771and back-facet mirror 772, a back-facet monitor photodiode 773, a laserdriver chip “LD” 774, a high-speed photodetector 763, a transimpedanceamplifier 764, a receiver chip “RX” 765, a heater stage “TEC/HT” 779 inthermal contact with GMC 770, and a controller “CNT” 775. The electricalports comprise receiver output (downstream data output) 766 which may bedifferential, transmitter data input (upstream data input) 777, andmodule diagnostics port 778 coupled to the controller 775 which may sendtraffic in either direction. The controller 775 monitors the temperatureusing a thermocouple or thermistor via line 792; the temperature sensoris denoted with the dot at the end of the line 792. The controller 775controls the temperature of the GMC 760 by energizing the heater orthermoelectric cooler 779. The controller 775 furthermore communicateswith the laser driver chip 774 via line 776: it sets the desired monitorphotodiode current I_(PD) or GMC output power and sends control signalsto the laser driver, such as, laser off and on, and receives theinformation on the magnitude of the drive current I_(LD) and diagnosticsignals, such as, for example, laser fault, over heating, and so on.

The transceiver is operatively configured to receive and emit opticalsignals through the optical fiber port 761 and electrical data andcontrol signals through electrical ports 766, 777, and 778. Opticalsignals with wavelengths in the downstream wavelength range are incidentthrough the optical fiber power 761 are routed by the duplexer 762 tothe high-speed photodetector 763 where the optical intensity isconverted to electrical current and then converted to voltage at thetrans-impedance amplifier 764. The analog signal at the exit of thetrans-impedance amplifier 764 is amplified and further processed in thereceiver 765 before it is routed to the electrical ports 766 associatedwith the transceiver electrical output. The photodetector 763,transimpedance-amplifier 764, and the receiver 765 constitute thereceiving chain of transmitter 760.

The GMC 770 is driven directly using a laser driver 774 which brings DCbias (operating point) and the AC modulation according to the data input777 as is well known in the art. The current output from the monitorphotodiode 773 coupled to the GMC is typically fed to the laser driveras input to the analog feedback loop that maintains the laser outputconstant over time and temperature.

The longitudinal mode separation Δf_(GMC) of the GMC 770 is designed toensure that only one mode ever appears within the passband of thechannel of the external AWG: Δf_(GMC)≧FWHM.

The transceiver 760 further comprises a beam splitter 781 which ispositioned between the front facet 771 of the GMC 770 and the opticalport 761 and incident-monitor photodiode 782. The beam-splitter 781couples a fraction of the optical power emitted P₂ from the GMC 770 tothe incident-monitor photodiode 782. The output from the monitorphotodiode 782 is fed to the controller 775.

In one embodiment, the mode tuning in a WDM-PON transmitter architectureshown in FIG. 7B comprises measuring the photodetector current I_(PD)and simultaneously adjusting the GMC drive current to maintain theI_(PD) at a preset value. The controller further periodically dithersthe heater drive power, measures a periodic oscillation in GMCtemperature and the resulting periodic oscillation in the emitted powerP₂. The controller is operatively configured to use the time evolutionof emitted power P₂ and the temperature T_(GMC) to maintain the averageGMC temperature T _(GMC) at a value for which the emitted power P₂ isminimized. In this way, one longitudinal mode of the GMC will remainwithin the passband of the array-waveguide array regardless of theambient temperature. The case of the transceiver 760 is assumed to be atan ambient temperature T_(A) which is generally different from the GMC770 temperature T_(GMC).

X. Real-Time Mode-Tuning Method

The mode adjustment approaches disclosed in this application rely onfinding the value of GMC temperature that corresponds to a maximum, aminimum, or a value in between of a response (current or optical power)versus temperature. There is more than one way of implementing this typeof control. This application discloses one possible way.

X.1 One Embodiment of a Method for Mode Tuning

In this embodiment, the power drive to the heater or the thermoelectriccooler is alternated between two values that correspond to twotemperatures separated by a small value, for example, 1° C. Namely, thepower drive to the heater/TEC is a periodically modulating signal withpeak-to-peak magnitude ΔP_(HT) superimposed on a constant value P _(HT).The periodic function is arbitrary, but in this implementation asquare-wave signal with repetition period t_(HT) is used. The constantdrive P _(HT) results in an average temperature T _(GMC) of the GMC,while the peak-to-peak value ΔP_(HT) and the period t_(HT) of thesquare-wave modulation determines the shape and the peak-to-peakamplitude of the temperature oscillation ΔT_(GMC)(t). If the period ofthe square-wave t_(HT) is set to be comparable to the thermal timeconstant τ_(HT) of the GMC and the package, the result will be a smoothnear-sinusoidal variation in ΔT_(GMC)(t). Adjusting the period t_(HT),shape and the amplitude ΔP_(HT) of the heating power to producesinusoidal, square-wave or other temperature variation does not departfrom the invention.

The temperature oscillation T_(GMC)(t) produces an oscillatingwavelength shift and through it a change in the response (laser drivecurrent or measured optical power) is then detected. Based on the datashown in FIGS. 5A and 5B, stabilization and locking of a mode to the AWGchannel passband in a self-seeded (FIG. 5A) or BLS-seeded (FIG. 5B)system is based on finding T_(GMC) for which an extremum is exhibited inthe response. The response, denoted with R(T_(GMC)), is either the laserdrive current R(T_(GMC))=I_(LD)(T_(GMC)) or measured optical powerR(T_(GMC))=P_(X)(T_(GMC)), where X=1 for self-seeded and X=2 forBLS-seeded system. As shown in FIG. 5A, in a self-seeding system, themaximum in the output power P₄ does not exactly correspond to a minimumin I_(LD). In this case, it is necessary to search for a specificpredetermined slope in the I_(LD)(T_(GMC)) relationship rather than forthe extremum where slope equals zero. This type of control can beimplemented in the same manner as searching for zero slope one. Thisembodiment will be disclosed below.

The second control loop includes evaluating the slope of the responseR(T_(GMC)) where one needs to dither the instantaneous temperatureT_(GMC)(t) around an average temperature T _(GMC) set point, monitor thetime evolution of the response R(t), and observe the correlation C_(RT)between the instantaneous temperature differenceΔT_(GMC)(t)=T_(GMC)(t)−T _(GMC) and the instantaneous responsedifference ΔR(t)=R(t)−R(T _(GMC)). The correlation is performed in realtime by integrating/summing the product C_(RT)=ΣΔT(t)ΔR(t) over acertain amount of time. From the correlation C_(RT) it isstraightforward to estimate the temperature for which R(T_(GMC)) is atan extremum or has a slope: When C_(RT)>0, the response grows withtemperature, and when C_(RT)<0, it reduces. Due to the presence of noisein the measurement, one may preset a boundary value for the correlation|C_(ET)|≦C_(EXT), for which we shall treat the slope as beingsubstantially equal to zero. If one were seeking a maximum, the secondcontrol loop should be designed so that when C_(RT)≧C_(EXT), the heaterdrive is increased to step up the value of T _(GMC), ifC_(RT)≦−|C_(EXT), the heater drive is decreased to step down the valueof T _(GMC), and if |C_(RT)|≦C_(EXT), the heater drive remainsunchanged. If we are seeking for a minimum, the second control loopshould be designed so that when C_(RT)≧C_(EXT), the heater drive isdecreased to step down the value of T _(GMC), if C_(RT)≧−|C_(EXT), theheater drive is increased to step up the value of T _(GMC), and if|C_(RT)|≦C_(EXT), the heater drive remains unchanged. Note that C_(EXT)may be zero. Additionally, one can also use this method to lock responseR at an arbitrary slope by forcing the second loop to maintain atemperature T _(GMC) where the correlation has a fixed positive ornegative value C₀, namely, |C_(RT)+C₀|≦C_(EXT). This type of controlloop is applicable to mode tuning in self-seeded systems using GMC drivecurrent I_(LD) or return optical power P₁, as well as mode tuning inBLS-seeded systems using GMC drive current I_(LD) or emitted opticalpower P₂.

X.2 Circuit Implementation of Method for Mode Tuning

The above-disclosed method for mode tuning may be implemented usinganalog or digital control as is well known in the art of automaticcontrol. FIG. 8 illustrates one possible implementation of the controlcircuits 800 that perform both the first and the second control loops.

The control circuit 800 comprises a microprocessor 801 which features anumber of inputs, noted A-to-D converters (ADC), and outputs, noted asD-to-A converters (DAC). One of the outputs allows pulse-widthmodulation output noted as PWMOUT. This type of controller or similarcontroller are available publicly, the unit used in this implementationis manufactured by NXP Semiconductors headquartered in Eindhoven, TheNetherlands. Fabry-Pérot laser 802 with anti-reflective coating on itsfront facet is the GMC co-packaged with a monitor photodiode 803 and inthermal contact with a resistive heater 804 and a thermometer 805. Inthis implementation, temperature measurement is performed using atemperature sensor LM35 (805) manufactured by Texas Instruments, Dallas,Tex. The thermometer 805, the heater 804, the GMC 802, and the monitorphotodiode 803 are co-located and in thermal contact illustrated withthe box 806. A portion of the light emitted by the Fabry-Pérot laser 802is coupled into an optical fiber 808 and a portion is captured by themonitor photodiode 803. The GMC 802 is powered by a 3.3-V power supply(VCC) and the current I_(LD) through it controlled by NPN transistor 807whose base bias is controlled by an output from the controller 801(through a series 10 kohm resistor). The current I_(LD) passing throughthe GMC 802 is measured by the controller 801 as voltage drop across the20-ohm resistor in the emitter of the transistor 807. The monitorphotodiode is powered using a 3.3-V power supply VCC and itsphotocurrent I_(PD) measured by the controller 801 as voltage dropacross a 2-kohm resistor. Note that this is a simple conversion ofcurrent to voltage using a resistor, where one could use a separatetransimpedance amplifier in the place of the resistor. The temperatureT_(GMC) of the laser 802 is measured by the temperature sensor 805 andits output is fed to the controller 801. The current through theresistive heater 804 heating is controlled by transistor 810 whose baseis driven by the pulse-width modulation output of the controller 801.The resistive heater 804 in this implementation has resistance 10 ohms.The measurements of optical power P₁ and P₂ are fed to one of the analoginputs on the controller and indicated with 814. The data input 813 andthe DC bias (via transistor 807) are brought to the GMC 802 via abias-tee comprising of a capacitor 812 and an inductor 811.

In this architecture the microprocessor 801 manages both control loops:the first loop comprises of measuring the current I_(PD) through thephotodetector 803, comparing it to a reference I_(PD0) set internallywithin the microprocessor and adjusting the laser drive current I_(LD)in order to make the difference between the preset value I_(PD0) and themeasured value I_(PD) substantially zero. The second loop comprises ofcontrolling the drive to the heater 804: providing, using pulse-widthmodulated signal, an average heating power P _(HT) with a square-waveheating power modulation ΔP_(HT)(t) with a specified square-waverepetition period. The average heating power results in the GMC averagetemperature T _(GMC), while the square-wave modulation results in asmooth and periodic temperature variation ΔT_(GMC)(t)=T_(GMC)(t)−T_(GMC) superimposed on top of the average temperature T _(GMC). Thecontroller 801 controls the temperature dither T_(GMC)(t), measures theresponse R(t), performs the correlation computation C_(RT), and thenmakes appropriate corrections to the average temperature T _(GMC) asdescribed in section VIII.1.

X.3 Mode Tuning Principle Illustrated on Measured Data

In one embodiment, the controller circuit 800 is inserted into theself-seeded system shown in FIG. 4A and is contained in the box 404. Inone embodiment, an output from a photodetector circuit (not shown)detecting return optical power P₁ is connected to the terminal 814 ofthe microprocessor 801. The experimental results obtained on thisimplementation are shown in FIG. 9. In another embodiment, thecontroller circuit 800 is inserted into the BLS-seeded system 450 shownin FIG. 4B and is contained in box 454. In one embodiment, an outputfrom a photodetector circuit (not shown) detecting emission opticalpower P₂ is connected to the terminal 814 of the microprocessor 801.

In FIG. 9, the horizontal axis is time and the vertical axis shows theGMC current I_(LD)(t), the output power P₄(t), the reflected power NO,the GMC temperature T_(GMC)(t), and the heating power P_(HT)(t). Thequantities have been scaled to fit on the same chart, hence the unitsare arbitrary. The monitor photodetector current I_(PD) was keptconstant throughout. The heating power P_(HT)(t) varies according to asquare wave around an average value P _(HT). The heating powermodulation ΔP_(HT)(t) is a square-wave with an approximate modulationperiod of 50 seconds. The average heating power P _(HT) changed from theinitial value in time interval 904 to a higher value during interval907. The resulting T_(GMC)(t) has a near sinusoidal variation which canbe described as an average temperature T _(GMC) with modulationΔT_(GMC)(t), the average temperature T _(GMC) being different in the twotime intervals (904 and 907) with different heating power: interval 907the temperature T _(GMC) is slightly higher than in interval 904. Thisincrease in the average temperature T _(GMC) is responsible for thereductions in the average output power P ₄, average reflected power P ₁,and average drive current Ī_(LD). The bar on the variables means thatthese values are averaged over the square-wave period of the heatingdrive.

The behavior of the measured quantities in FIG. 9 is explained with thehelp of FIG. 5A: The average temperature T _(GMC) during interval 904approximately corresponds to the temperature 513, while the averagetemperature T _(GMC) during interval 907 approximately corresponds to atemperature slightly above temperature 514 in FIG. 5A. It is clear thatan increase in temperature T _(GMC) just above the point 512 will makethe correlation between the oscillatory part of the drive currentI_(OSC)(t) and the temperature T_(GMC)(t) positive (the currentincreases with temperature). This is confirmed in FIG. 9 where duringthe time interval 907 the small positive bumps in I_(LD)(t) arecorrelated with the temperature. However, during the interval 904, attime 903 it is visible that the correlation between I_(OSC)(t) andT_(GMC)(t) is negative.

If we were trying to find the minimum of the drive current I_(OSC)(t),detecting negative correlation in interval 904 would cause themicroprocessor to slightly increase the average heater drive P _(HT) inan attempt to reach a point where the I_(OSC) is at a minimum—theinterval 907 is evidently very close if not exactly at that temperature.On the other hand, if we were using P₁ to correct the second loop, thenduring interval 907 the microprocessor would detect a negativecorrelation between the temperature and the return power P₁. This wouldcause the microprocessor to reduce the average heater drive P _(HT) inorder to reach a point where the P₁ is maximized. Interval 904, wherethe temperature is reduced, exhibits higher average P₁ value, but alsoshows an onset of saturation at the top of the near-sinusoidal signalindicating that the temperature setting T _(GMC) is close to the maximalvalue of P₁, namely, close to the GMC temperature that sets thelongitudinal mode of the Fabry-Perot laser exactly at the center of theAWG passband.

FIG. 9 illustrates the behavior of the measured quantities used in thesecond control loop for tuning the GMC modes in a self-seeded system.The approach was described for both the embodiment in which the secondloop searches for the extremum or a specific slope in the laser drivecurrent, and the for the embodiment in which the second loop searchesfor the extremum in the return optical power P₁. For BLS-seeded systems,the method is analogous except that the loop searches for the minimum inthe outgoing optical power P₂.

It is clear that control loops that find and maintain the response at apredetermined slope can be implemented in multiple ways. Therefore, thepresented approach using digital control loop and dithering thetemperature is one non-limiting example of the first and second controlloops managed using one microprocessor. Present-day laser drivercircuits generally include an analog loop for adjusting the laser outputbased on the current from the monitor photodiode. In one embodiment, thelaser driver includes the automatic output-power control, while amicroprocessor/controller manages the second control loop, namely,adjusts the temperature and finds the slope in the response function:the laser drive current I_(LD) or the optical powers P₁ or P₂.

The invention claimed is:
 1. A wavelength-division multiplexed opticalsource comprising an optical chip operatively configured to provide again spectrum with a multiplicity of resonances characterized with modewavelengths and a mode wavelength separation when a drive current ispassed through said optical chip, said mode wavelengths dependent on aoptical-chip temperature; an optical bandpass filter having at least onedistribution port and at least one common port, and having a passbandwith a center wavelength and a full-width at half maximum (FWHM)bandwidth expressed in wavelength; a segment of distribution opticalfiber having a remote-node end optically coupled to said at least onedistribution port and having a client end optically coupled to saidoptical chip; a first monitor photodiode optically coupled to saidoptical chip and operatively configured to pass first monitor current inproportion to an intensity of light incident on said first monitorphotodiode; a reflector assembly having a reflection port and atransmission port, said reflection port optically coupled to said commonport; wherein said mode wavelength separation is larger than said FWHMbandwidth.
 2. The optical source of claim 1, wherein said opticalbandpass filter is an athermal array-waveguide grating.
 3. The opticalsource of claim 1, wherein said reflector assembly comprises amodulation averaging structure.
 4. The optical source of claim 1,wherein said reflector assembly comprises a Faraday Rotating Mirror. 5.The optical source of claim 1 further comprising a driver chipoperatively configured to provide and adjust said drive current tomaintain said first monitor current at a constant predetermined monitorcurrent value; a controller chip; a cooling/heating stage and athermometer component both in thermal contact with said optical chip andelectrically connected to said controller chip, said thermometeroperatively configured to provide optical chip temperature to saidcontroller chip and said controller chip is operatively configured tocontrol said optical chip temperature to a temperature value; whereinfor said temperature value only one mode wavelength appears within saidpassband.
 6. The optical source of claim 5, wherein for said temperaturevalue a drive-current versus optical-chip temperature characteristicexhibits a predetermined slope.
 7. The optical source of claim 6,wherein said optical bandpass filter is an athermal array-waveguidegrating.
 8. The optical source of claim 6, wherein said reflectorassembly comprises a modulation averaging structure.
 9. The opticalsource of claim 6, wherein said reflector assembly comprises a FaradayRotating Mirror.
 10. The optical source of claim 1 further comprising adriver chip operatively configured to provide and adjust said drivecurrent to maintain said first monitor current at a constantpredetermined monitor current value; a second monitor photodiodeoptically coupled said client end of distribution fiber, said secondmonitor photodiode operatively configured to pass second monitor currentin proportion to optical power incident on said second monitorphotodiode; a controller chip; a cooling/heating stage and a thermometercomponent both in thermal contact with said optical chip andelectrically connected to said controller chip, said thermometeroperatively configured to provide optical chip temperature to saidcontroller chip and said controller chip is operatively configured tocontrol said optical chip temperature to a temperature value; whereinfor said temperature value only one mode wavelength appears within saidpassband.
 11. The optical source of claim 10, wherein for saidtemperature value second monitor current versus optical chip temperaturecharacteristic exhibits a maximum.
 12. The optical source of claim 11,wherein said optical bandpass filter is an athermal array-waveguidegrating.
 13. The optical source of claim 11, wherein said reflectorassembly comprises a modulation averaging structure.
 14. The opticalsource of claim 11, wherein said reflector assembly comprises a FaradayRotating Mirror.
 15. A wavelength-division multiplexed optical sourcecomprising an optical chip operatively configured to provide a gainspectrum with a multiplicity of resonances characterized with modewavelengths and a mode wavelength separation when a drive current ispassed through said optical chip, said mode wavelengths dependent on aoptical-chip temperature; an optical bandpass filter having at least onedistribution port and at least one common port, and having a passbandwith a center wavelength and a full-width at half maximum (FWHM)bandwidth expressed in wavelength; a monitor photodiode opticallycoupled to said optical chip and operatively configured to pass monitorcurrent in proportion to an intensity of light incident on said monitorphotodiode; a segment of distribution optical fiber having a remote-nodeend optically coupled to said at least one distribution port and havinga client end optically coupled to said waveguide; a broadband lightsource optically coupled to said at least one common port; wherein saidmode wavelength separation is larger than said FWHM bandwidth.
 16. Theoptical source of claim 15 further comprising a driver chip operativelyconfigured to provide and adjust said drive current to maintain saidmonitor current at a constant predetermined monitor current value; acontroller chip; a cooling/heating stage and a thermometer componentboth in thermal contact with said optical chip and electricallyconnected to said controller chip, said thermometer operativelyconfigured to provide optical chip temperature to said controller chipand said controller chip operatively configured to control said opticalchip temperature to a temperature value; wherein for said temperaturevalue only one mode wavelength appears within said passband.
 17. Theoptical source of claim 16, wherein for said temperature value a drivecurrent versus optical chip temperature characteristic exhibits aminimum.
 18. The optical source of claim 17, wherein said opticalbandpass filter is an athermal array-waveguide grating.